In an apparatus formed by mounting an optical microscope or exposure X-Y stage on an anti-vibration table, externally transmitted vibration must be eliminated as much as possible. An apparatus in which an exposure X-Y stage is mounted on an anti-vibration table has intermittent operations called step & repeat or step & scan as a driving mode. It should be noted that the apparatus itself generates vibration because of repeated step or scan, thus vibrating the anti-vibration table. Unless this vibration is eliminated, exposure cannot be performed. The anti-vibration table is therefore required to realize anti-vibration against external vibration and vibration control against forced vibration caused by the motion of the equipment itself mounted on the anti-vibration table with a good balance.
Anti-vibration tables are classified into passive anti-vibration tables and active anti-vibration tables. Recently, to meet demands such as high-precision positioning, high-precision scanning, high-speed movement, and the like, for the equipment mounted on the anti-vibration table, an active anti-vibration apparatus tends to be used. An example of an actuator for driving the anti-vibration table includes a pneumatic spring, a voice coil motor, a piezoelectric element, and the like.
FIG. 7 shows the arrangement of Japanese Patent Laid-Open No. 11-264444 disclosing a conventional active anti-vibration apparatus.
Referring to FIG. 7, reference numerals 1 and 8 denote pneumatic spring type support legs; 2 and 9, servo valves for supplying and exhausting air as the working fluid to and from pneumatic springs 3 and 10; 4 and 11, position sensors for measuring the vertical displacement of an anti-vibration table 15 at their measurement points; 5 and 12, mechanical springs for pre-pressurizing; 6, a viscous element for expressing the viscosities of the pneumatic spring 3, mechanical spring 5, and pneumatic spring type support leg 1; and 13, a viscous element for expressing the viscosities of the pneumatic spring 10, mechanical spring 12, and pneumatic spring type support leg 8.
Reference numerals 7 and 14 denote acceleration sensors; 16, a stage which moves in the horizontal direction on the anti-vibration table 15; 17, a position sensor for measuring the horizontal displacement of the stage 16; 18, a motor for driving the stage 16; 19, a displacement amplifier for amplifying a stage displacement; 20, a PID compensator; 21, an amplifier; 22, a target speed generator for generating the target speed of the stage 16 on the basis of an operation profile generated by a stage sequence controller 37; and 23, an integrator, respectively.
Reference numerals 24 and 25 denote displacement amplifiers; 26, a displacement decomposer; 27 and 28, PI compensators; 29 and 30, filters; 31, an acceleration decomposer; 32, a thrust distributor; and 33 and 34, amplifiers, respectively.
Reference numeral 35 denotes a target position setter for generating the target position of the anti-vibration table 15; and 36, a filter with a predetermined appropriate gain and time constant.
The above constituent elements 1 to 18 in FIG. 7 are schematically illustrated as the active anti-vibration apparatus is seen just from its side. The pneumatic spring type support legs 1 and 8 support the anti-vibration table 15 in the vertical direction.
The operation of the pneumatic spring type anti-vibration apparatus shown in FIG. 7 will be described. For the sake of convenience, the moving direction of the stage 16, the vertical direction, and an axis extending through the barycenter of the anti-vibration table 15 and perpendicular to the moving direction of the stage 16 within a horizontal plane are respectively defined as X-, Z-, and Y-axes. An axis of rotation about the Y-axis is defined as a θy-axis.
Outputs from the acceleration sensors 7 and 14 respectively pass through the filters 29 and 30, each having a predetermined appropriate gain and time constant, and are input to the acceleration decomposer 31. The acceleration decomposer 31 decomposes the two inputs into a Z-direction acceleration and a θy-direction angular acceleration by 2×2 multiply matrix operation, and negatively feeds them back to the input of the thrust distributor 32. This acceleration feedback loop adds damping.
Outputs from the position sensors 4 and 11 respectively pass through the displacement amplifiers 24 and 25 and are input to the displacement decomposer 26. The displacement decomposer 26 decomposes the two inputs into a Z-direction displacement and a θy-direction rotational displacement by 2×2 multiply matrix operation.
The target position setter 35 sets target positions for the vertical displacement and rotational displacement. Deviation signals of the target positions with respect to outputs from the displacement decomposer 26 pass through the PI compensators 27 and 28 and are input to the thrust distributor 32.
The thrust distributor 32 distributes Z- and θy-direction thrust target values to the driving target values of the pneumatic springs 3 and 10. The distributed driving target values are converted by the amplifiers 33 and 34 into driving currents for the servo valves 2 and 9, respectively. When the servo valves 2 and 9 are opened or closed, the pressures in the pneumatic springs 3 and 10 are adjusted. Thus, the anti-vibration table 15 can be held at the desired position set by the target position setter 35 without any steady deviation.
The PI compensators 27 and 28 operate as a control compensator for the Z-axis displacement and a control compensator for the θy-axis displacement, respectively.
An output from the position sensor 17 passes through the displacement amplifier 19, and its deviation with respect to a target position signal generated by the integrator 23 is input to the PID compensator 20. An output from the PID compensator 20 passes through the amplifier 21 and drives the stage 16 through the motor 18. P, I, and D of the PID compensator 20 mean proportional, integral, and derivative, respectively. The target speed generator 22 generates the target speed of the stage 16 on the basis of the operation profile generated by the stage sequence controller 37. The target speed of the stage 16 is integrated by the integrator 23 to become the target position of the stage 16. The target speed of the stage 16 is feed-forwarded to a θy-axis displacement control system through the filter 36.
The intermediate portion of FIG. 8 is a graph showing the target speed generated by the target speed generator 22 by plotting the time along the axis of abscissa. This speed pattern is known as a trapezoidal speed pattern. The upper and lower portions of FIG. 8 show a target acceleration as the differential value of the target speed and a target position as the integral value of the target speed. The target speed is converted into the target position through the integrator 23, and is input to a position control system for the stage 16.
It is generally known that, when the pneumatic springs 3 and 10 are in the equilibrium state, a transfer function from an input current I of the servo valves 2 and 9 to a pressure P of the pneumatic springs 3 and 10 can be approximated by integral characteristics as the following equation (1):                               P          I                =                  Gq          ⁢                                           ⁢                                    kP              0                                                      V                0                            ⁢              s                                                          (        1        )            
where Gq is the flow rate gain of the servo valve, k is the specific heat of air, P0 is the pressure of the pneumatic spring in the equilibrium state, V0 is the volume of the pneumatic spring in the equilibrium state, and s is the Laplacian operator.
Therefore, the target speed feed-forwarded to the rotational displacement control system of the anti-vibration table 15 is integrated by the integral characteristics of the pneumatic spring 10 of equation (1) Feed-forwarding the target value is equivalent to giving the anti-vibration table 15 a rotational torque proportional to the position of the stage 16. Feed-forwarding of the target speed can cancel the rotational moment generated by the movement of the stage 16. The vibration of the anti-vibration table 15 can thus be effectively suppressed.
Japanese Patent Laid-Open No. 11-72136 discloses that a change in barycenter of the whole anti-vibration table 15 is obtained from the position of the stage 16, and that the pressure of the pneumatic spring is adjusted in accordance with this change.
In a recent exposure apparatus used in the manufacture of a device such as a semiconductor device, to improve the exposure throughput, the moving speed of the stage becomes high. Particularly, when, e.g., the moving speed of a movable portion such as the stage exceeds the pressure change speed of the pneumatic spring, the pneumatic spring cannot follow its driving signal, and sometimes compensation of the moving load by means of the pneumatic spring undesirably vibrates the anti-vibration table.